The invention relates to high voltage stable and high density lithium metal oxide powderous compounds, containing a core having stoichiometrically controlled lithium content and an electron insulating surface. The compounds may comprise known elements such as Mg, Ti and Al for obtaining improved high voltage electrochemical performances and improved energy density. Also a method to manufacture these materials is disclosed. The lithium transition metal oxide powder can be used as a cathode active material in rechargeable lithium batteries.
Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras. Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO2-based cathode materials. As today's consumer electronics demand rechargeable batteries with higher energy density, there is a surge towards LiCoO2-based materials with increased specific capacity for more demanding end applications.
Two common ways to improve the energy density are (a) to increase the charge voltage, typically 4.5V or even 4.6V vs. Li metal when fitted in coin cells, and 4.35V and 4.4V vs. graphite when fitted in full cells, which requires more robust cathode materials which can be charged at higher voltage and (b) to increase the packing density, which requires to increase the particle size of the powder particles. Industrial applicability of these two approaches is however limited by side problems. On one hand, increasing the charge voltage leads to unstable behavior of the electrode, resulting in cathode degradation linked with electrolyte decomposition. As lithium is removed from LixCoO2 (x<1), oxidation of Co3+ to an unstable oxidation state Co4+ follows. The higher the charge voltage, the higher the amount of Co4+. Large concentrations of Co4+ increase unwanted side reactions between electrolyte and charged cathode. These side reactions result in poor safety, poor cycling stability at elevated voltage and of poor storage properties of charged cathode at elevated temperature. On the other hand, increasing the particle size to increase the packing density impairs power capabilities of rechargeable batteries. In order to meet the power requirements, the battery as a whole and particularly the active cathode material itself must have a sufficient high rate performance. Increasing the mean particle size reduces the solid-state lithium diffusion length which eventually results in lowered rate performance.
Careful studying of published results on cathode materials allows to better understanding the limitations of LiCoO2 based rechargeable lithium batteries. A fundamental limitation of state of the art LiCoO2-based materials development lies in the Li-excess and particle size dilemma. In WO2010-139404, the authors illustrate the relationship between packing density, mean particle size and lithium excess used for the preparation of state of the art Mg and Ti doped LiCoO2. In short, the higher the packing density, the higher the particle size and the higher the Li-excess, expressed as Li:Co>>1.00—typically Li:Co is around 1.05—used for the synthesis. The mechanism is based on a so-called “lithium-flux effect” where the Li-excess acts as a flux enhancing the growth of LiCoO2 particles which eventually increases the packing density. Typical packing densities of ca. 3.70 g/cm3 are achieved for 18 μm particles. Authors also emphasize that large pressed densities are preferable and obtained with monolithic, potato-shaped and non agglomerated primary LiCoO2 particles. Use of larger Li:Co excesses to achieve larger monolithic particles results however in poor electrochemical performances, with lower C-rate and lower discharge capacity, which in return cancels energy density gains achieved by increasing the particle size. Such large Li:Co values also increase pH, free base content and carbon content, which impairs safety, storage and bulging properties of charged cathodes. Levasseur, in Chem. Mater., 2002, 14, 3584-3590 establishes a clear relationship between the increase of structural defect concentrations, as evidenced by means of 7Li MAS NMR, and the increase of Li:Co excess.
As a consequence, current state of the art synthesis does not allow to achieve dense, monolithic LiCoO2-based particles with reduced Li:Co excess. Partial improvements have been achieved but the above basic problems have not yet been fully resolved. Hence there is clearly a need for high capacity LiCoO2 based cathodes which can be cycled in a stable manner in real cells at higher voltages.
In the prior art several approaches have been suggested to cope with this problem. To achieve high voltage stability, LiCoO2 materials are usually coated (for example with Al2O3) or otherwise chemically modified (e.g. by providing a fluorinated surface). A problem is that coated dense LiCoO2 often has a lower reversible capacity, so that a part of the gain of energy density by charging to higher voltage is annulled by lower intrinsic capacity. This effect can be observed for aluminum oxide protective and LiF protective coatings, but similar effects are observed for other coating approaches such as ZrO2, AlPO4 . . . .
Studying the literature furthermore tells us that coating might not be necessary at all to achieve high voltage stability. ChenDahn (Electrochem. Solid-State Lett., Volume 7, Issue 1, pp. A11-A14 (2004)) for example report that a fresh prepared LiCoO2 could cycle in a stable manner at 4.5V if tested in coin cells with Li metal anodes. Such an approach might be correct for coin cells but the effect cannot be reproduced in real commercial cells. These results are confirmed by the fact that now, several years after the publication, special treated—and not pure—LiCoO2 is commercially sold for high voltage applications.
Currently no other strategies are known which lead to high voltage performances. It is an object of the present invention to define a cathode material having a high packing density, high rate performance, improved discharge capacity and showing high stability during extended cycling at high charge voltage for high end secondary battery applications.